| Message ID | 1400492771-9746-1-git-send-email-acourbot@nvidia.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
Am Montag, den 19.05.2014, 18:46 +0900 schrieb Alexandre Courbot: > This patch is not meant to be merged, but rather to try and understand > why this is needed and what a more suitable solution could be. > > Allowing BOs to be write-cached results in the following happening when > trying to run any program on Tegra/GK20A: > > Unhandled fault: external abort on non-linefetch (0x1008) at 0xf0036010 > ... > (nouveau_bo_rd32) from [<c0357d00>] (nouveau_fence_update+0x5c/0x80) > (nouveau_fence_update) from [<c0357d40>] (nouveau_fence_done+0x1c/0x38) > (nouveau_fence_done) from [<c02c3d00>] (ttm_bo_wait+0xec/0x168) > (ttm_bo_wait) from [<c035e334>] (nouveau_gem_ioctl_cpu_prep+0x44/0x100) > (nouveau_gem_ioctl_cpu_prep) from [<c02aaa84>] (drm_ioctl+0x1d8/0x4f4) > (drm_ioctl) from [<c0355394>] (nouveau_drm_ioctl+0x54/0x80) > (nouveau_drm_ioctl) from [<c00ee7b0>] (do_vfs_ioctl+0x3dc/0x5a0) > (do_vfs_ioctl) from [<c00ee9a8>] (SyS_ioctl+0x34/0x5c) > (SyS_ioctl) from [<c000e6e0>] (ret_fast_syscall+0x0/0x30 > > The offending nouveau_bo_rd32 is done over an IO-mapped BO, e.g. a BO > mapped through the BAR. > Um wait, this memory is behind an already mapped bar? I think ioremap on ARM defaults to uncached mappings, so if you want to access the memory behind this bar as WC you need to map the BAR as a whole as WC by using ioremap_wc. Regards, Lucas > Any idea about the origin of this behavior? Does ARM forbid cached > mappings over IO regions? > > Signed-off-by: Alexandre Courbot <acourbot@nvidia.com> > --- > drivers/gpu/drm/nouveau/nouveau_bo.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/drivers/gpu/drm/nouveau/nouveau_bo.c b/drivers/gpu/drm/nouveau/nouveau_bo.c > index 8db54a217232..9cfb8e61f5c4 100644 > --- a/drivers/gpu/drm/nouveau/nouveau_bo.c > +++ b/drivers/gpu/drm/nouveau/nouveau_bo.c > @@ -552,7 +552,11 @@ nouveau_bo_init_mem_type(struct ttm_bo_device *bdev, uint32_t type, > TTM_MEMTYPE_FLAG_MAPPABLE; > man->available_caching = TTM_PL_FLAG_UNCACHED | > TTM_PL_FLAG_WC; > +#if defined(__arm__) > + man->default_caching = TTM_PL_FLAG_UNCACHED; > +#else > man->default_caching = TTM_PL_FLAG_WC; > +#endif > break; > case TTM_PL_TT: > if (nv_device(drm->device)->card_type >= NV_50)
On 05/19/2014 06:57 PM, Lucas Stach wrote: > Am Montag, den 19.05.2014, 18:46 +0900 schrieb Alexandre Courbot: >> This patch is not meant to be merged, but rather to try and understand >> why this is needed and what a more suitable solution could be. >> >> Allowing BOs to be write-cached results in the following happening when >> trying to run any program on Tegra/GK20A: >> >> Unhandled fault: external abort on non-linefetch (0x1008) at 0xf0036010 >> ... >> (nouveau_bo_rd32) from [<c0357d00>] (nouveau_fence_update+0x5c/0x80) >> (nouveau_fence_update) from [<c0357d40>] (nouveau_fence_done+0x1c/0x38) >> (nouveau_fence_done) from [<c02c3d00>] (ttm_bo_wait+0xec/0x168) >> (ttm_bo_wait) from [<c035e334>] (nouveau_gem_ioctl_cpu_prep+0x44/0x100) >> (nouveau_gem_ioctl_cpu_prep) from [<c02aaa84>] (drm_ioctl+0x1d8/0x4f4) >> (drm_ioctl) from [<c0355394>] (nouveau_drm_ioctl+0x54/0x80) >> (nouveau_drm_ioctl) from [<c00ee7b0>] (do_vfs_ioctl+0x3dc/0x5a0) >> (do_vfs_ioctl) from [<c00ee9a8>] (SyS_ioctl+0x34/0x5c) >> (SyS_ioctl) from [<c000e6e0>] (ret_fast_syscall+0x0/0x30 >> >> The offending nouveau_bo_rd32 is done over an IO-mapped BO, e.g. a BO >> mapped through the BAR. >> > Um wait, this memory is behind an already mapped bar? I think ioremap on > ARM defaults to uncached mappings, so if you want to access the memory > behind this bar as WC you need to map the BAR as a whole as WC by using > ioremap_wc. Tried mapping the BAR using ioremap_wc(), but to no avail. On the other hand, could it be that VRAM BOs end up creating a mapping over an already-mapped region? I seem to remember that ARM might not like it...
Am Montag, den 19.05.2014, 19:06 +0900 schrieb Alexandre Courbot: > On 05/19/2014 06:57 PM, Lucas Stach wrote: > > Am Montag, den 19.05.2014, 18:46 +0900 schrieb Alexandre Courbot: > >> This patch is not meant to be merged, but rather to try and understand > >> why this is needed and what a more suitable solution could be. > >> > >> Allowing BOs to be write-cached results in the following happening when > >> trying to run any program on Tegra/GK20A: > >> > >> Unhandled fault: external abort on non-linefetch (0x1008) at 0xf0036010 > >> ... > >> (nouveau_bo_rd32) from [<c0357d00>] (nouveau_fence_update+0x5c/0x80) > >> (nouveau_fence_update) from [<c0357d40>] (nouveau_fence_done+0x1c/0x38) > >> (nouveau_fence_done) from [<c02c3d00>] (ttm_bo_wait+0xec/0x168) > >> (ttm_bo_wait) from [<c035e334>] (nouveau_gem_ioctl_cpu_prep+0x44/0x100) > >> (nouveau_gem_ioctl_cpu_prep) from [<c02aaa84>] (drm_ioctl+0x1d8/0x4f4) > >> (drm_ioctl) from [<c0355394>] (nouveau_drm_ioctl+0x54/0x80) > >> (nouveau_drm_ioctl) from [<c00ee7b0>] (do_vfs_ioctl+0x3dc/0x5a0) > >> (do_vfs_ioctl) from [<c00ee9a8>] (SyS_ioctl+0x34/0x5c) > >> (SyS_ioctl) from [<c000e6e0>] (ret_fast_syscall+0x0/0x30 > >> > >> The offending nouveau_bo_rd32 is done over an IO-mapped BO, e.g. a BO > >> mapped through the BAR. > >> > > Um wait, this memory is behind an already mapped bar? I think ioremap on > > ARM defaults to uncached mappings, so if you want to access the memory > > behind this bar as WC you need to map the BAR as a whole as WC by using > > ioremap_wc. > > Tried mapping the BAR using ioremap_wc(), but to no avail. On the other > hand, could it be that VRAM BOs end up creating a mapping over an > already-mapped region? I seem to remember that ARM might not like it... Multiple mapping are generally allowed, as long as they have the same caching state. It's conflicting mappings (uncached vs cached, or cached vs wc), that are documented to yield undefined results. Regards, Lucas
On Mon, May 19, 2014 at 7:16 PM, Lucas Stach <l.stach@pengutronix.de> wrote: > Am Montag, den 19.05.2014, 19:06 +0900 schrieb Alexandre Courbot: >> On 05/19/2014 06:57 PM, Lucas Stach wrote: >> > Am Montag, den 19.05.2014, 18:46 +0900 schrieb Alexandre Courbot: >> >> This patch is not meant to be merged, but rather to try and understand >> >> why this is needed and what a more suitable solution could be. >> >> >> >> Allowing BOs to be write-cached results in the following happening when >> >> trying to run any program on Tegra/GK20A: >> >> >> >> Unhandled fault: external abort on non-linefetch (0x1008) at 0xf0036010 >> >> ... >> >> (nouveau_bo_rd32) from [<c0357d00>] (nouveau_fence_update+0x5c/0x80) >> >> (nouveau_fence_update) from [<c0357d40>] (nouveau_fence_done+0x1c/0x38) >> >> (nouveau_fence_done) from [<c02c3d00>] (ttm_bo_wait+0xec/0x168) >> >> (ttm_bo_wait) from [<c035e334>] (nouveau_gem_ioctl_cpu_prep+0x44/0x100) >> >> (nouveau_gem_ioctl_cpu_prep) from [<c02aaa84>] (drm_ioctl+0x1d8/0x4f4) >> >> (drm_ioctl) from [<c0355394>] (nouveau_drm_ioctl+0x54/0x80) >> >> (nouveau_drm_ioctl) from [<c00ee7b0>] (do_vfs_ioctl+0x3dc/0x5a0) >> >> (do_vfs_ioctl) from [<c00ee9a8>] (SyS_ioctl+0x34/0x5c) >> >> (SyS_ioctl) from [<c000e6e0>] (ret_fast_syscall+0x0/0x30 >> >> >> >> The offending nouveau_bo_rd32 is done over an IO-mapped BO, e.g. a BO >> >> mapped through the BAR. >> >> >> > Um wait, this memory is behind an already mapped bar? I think ioremap on >> > ARM defaults to uncached mappings, so if you want to access the memory >> > behind this bar as WC you need to map the BAR as a whole as WC by using >> > ioremap_wc. >> >> Tried mapping the BAR using ioremap_wc(), but to no avail. On the other >> hand, could it be that VRAM BOs end up creating a mapping over an >> already-mapped region? I seem to remember that ARM might not like it... > > Multiple mapping are generally allowed, as long as they have the same > caching state. It's conflicting mappings (uncached vs cached, or cached > vs wc), that are documented to yield undefined results. Sorry about the confusion. The BAR is *not* mapped to the kernel yet (it is BAR1, there is no BAR3 on GK20A) and an ioremap_*() is performed in ttm_bo_ioremap() to make the part of the BAR where the buffer is mapped visible. It seems that doing an ioremap_wc() on the BAR area on Tegra is what leads to these errors. ioremap() or ioremap_nocache() (which are in effect the same on ARM) do not cause this issue. The best way to solve this issue would be to not use the BAR at all since the memory behind these objects can be directly accessed by the CPU. As such it would better be mapped using ttm_bo_kmap_ttm() instead. But right now this is clearly not how nouveau_bo.c is written and it does not look like this can easily be done. :/
Am Freitag, den 23.05.2014, 16:10 +0900 schrieb Alexandre Courbot: > On Mon, May 19, 2014 at 7:16 PM, Lucas Stach <l.stach@pengutronix.de> wrote: > > Am Montag, den 19.05.2014, 19:06 +0900 schrieb Alexandre Courbot: > >> On 05/19/2014 06:57 PM, Lucas Stach wrote: > >> > Am Montag, den 19.05.2014, 18:46 +0900 schrieb Alexandre Courbot: > >> >> This patch is not meant to be merged, but rather to try and understand > >> >> why this is needed and what a more suitable solution could be. > >> >> > >> >> Allowing BOs to be write-cached results in the following happening when > >> >> trying to run any program on Tegra/GK20A: > >> >> > >> >> Unhandled fault: external abort on non-linefetch (0x1008) at 0xf0036010 > >> >> ... > >> >> (nouveau_bo_rd32) from [<c0357d00>] (nouveau_fence_update+0x5c/0x80) > >> >> (nouveau_fence_update) from [<c0357d40>] (nouveau_fence_done+0x1c/0x38) > >> >> (nouveau_fence_done) from [<c02c3d00>] (ttm_bo_wait+0xec/0x168) > >> >> (ttm_bo_wait) from [<c035e334>] (nouveau_gem_ioctl_cpu_prep+0x44/0x100) > >> >> (nouveau_gem_ioctl_cpu_prep) from [<c02aaa84>] (drm_ioctl+0x1d8/0x4f4) > >> >> (drm_ioctl) from [<c0355394>] (nouveau_drm_ioctl+0x54/0x80) > >> >> (nouveau_drm_ioctl) from [<c00ee7b0>] (do_vfs_ioctl+0x3dc/0x5a0) > >> >> (do_vfs_ioctl) from [<c00ee9a8>] (SyS_ioctl+0x34/0x5c) > >> >> (SyS_ioctl) from [<c000e6e0>] (ret_fast_syscall+0x0/0x30 > >> >> > >> >> The offending nouveau_bo_rd32 is done over an IO-mapped BO, e.g. a BO > >> >> mapped through the BAR. > >> >> > >> > Um wait, this memory is behind an already mapped bar? I think ioremap on > >> > ARM defaults to uncached mappings, so if you want to access the memory > >> > behind this bar as WC you need to map the BAR as a whole as WC by using > >> > ioremap_wc. > >> > >> Tried mapping the BAR using ioremap_wc(), but to no avail. On the other > >> hand, could it be that VRAM BOs end up creating a mapping over an > >> already-mapped region? I seem to remember that ARM might not like it... > > > > Multiple mapping are generally allowed, as long as they have the same > > caching state. It's conflicting mappings (uncached vs cached, or cached > > vs wc), that are documented to yield undefined results. > > Sorry about the confusion. The BAR is *not* mapped to the kernel yet > (it is BAR1, there is no BAR3 on GK20A) and an ioremap_*() is > performed in ttm_bo_ioremap() to make the part of the BAR where the > buffer is mapped visible. It seems that doing an ioremap_wc() on the > BAR area on Tegra is what leads to these errors. ioremap() or > ioremap_nocache() (which are in effect the same on ARM) do not cause > this issue. > It would be cool if you could ask HW, or the blob developers, if this is a general issue. The external abort is clearly the GPUs AXI client responding with an error to the read request, though I'm not clear where a WC read differs from an uncached one. > The best way to solve this issue would be to not use the BAR at all > since the memory behind these objects can be directly accessed by the > CPU. As such it would better be mapped using ttm_bo_kmap_ttm() > instead. But right now this is clearly not how nouveau_bo.c is written > and it does not look like this can easily be done. :/ Yeah, it sounds like we want this shortcut for stolen VRAM implementations. Regards, Lucas
On 05/23/2014 06:24 PM, Lucas Stach wrote: > Am Freitag, den 23.05.2014, 16:10 +0900 schrieb Alexandre Courbot: >> On Mon, May 19, 2014 at 7:16 PM, Lucas Stach <l.stach@pengutronix.de> wrote: >>> Am Montag, den 19.05.2014, 19:06 +0900 schrieb Alexandre Courbot: >>>> On 05/19/2014 06:57 PM, Lucas Stach wrote: >>>>> Am Montag, den 19.05.2014, 18:46 +0900 schrieb Alexandre Courbot: >>>>>> This patch is not meant to be merged, but rather to try and understand >>>>>> why this is needed and what a more suitable solution could be. >>>>>> >>>>>> Allowing BOs to be write-cached results in the following happening when >>>>>> trying to run any program on Tegra/GK20A: >>>>>> >>>>>> Unhandled fault: external abort on non-linefetch (0x1008) at 0xf0036010 >>>>>> ... >>>>>> (nouveau_bo_rd32) from [<c0357d00>] (nouveau_fence_update+0x5c/0x80) >>>>>> (nouveau_fence_update) from [<c0357d40>] (nouveau_fence_done+0x1c/0x38) >>>>>> (nouveau_fence_done) from [<c02c3d00>] (ttm_bo_wait+0xec/0x168) >>>>>> (ttm_bo_wait) from [<c035e334>] (nouveau_gem_ioctl_cpu_prep+0x44/0x100) >>>>>> (nouveau_gem_ioctl_cpu_prep) from [<c02aaa84>] (drm_ioctl+0x1d8/0x4f4) >>>>>> (drm_ioctl) from [<c0355394>] (nouveau_drm_ioctl+0x54/0x80) >>>>>> (nouveau_drm_ioctl) from [<c00ee7b0>] (do_vfs_ioctl+0x3dc/0x5a0) >>>>>> (do_vfs_ioctl) from [<c00ee9a8>] (SyS_ioctl+0x34/0x5c) >>>>>> (SyS_ioctl) from [<c000e6e0>] (ret_fast_syscall+0x0/0x30 >>>>>> >>>>>> The offending nouveau_bo_rd32 is done over an IO-mapped BO, e.g. a BO >>>>>> mapped through the BAR. >>>>>> >>>>> Um wait, this memory is behind an already mapped bar? I think ioremap on >>>>> ARM defaults to uncached mappings, so if you want to access the memory >>>>> behind this bar as WC you need to map the BAR as a whole as WC by using >>>>> ioremap_wc. >>>> >>>> Tried mapping the BAR using ioremap_wc(), but to no avail. On the other >>>> hand, could it be that VRAM BOs end up creating a mapping over an >>>> already-mapped region? I seem to remember that ARM might not like it... >>> >>> Multiple mapping are generally allowed, as long as they have the same >>> caching state. It's conflicting mappings (uncached vs cached, or cached >>> vs wc), that are documented to yield undefined results. >> >> Sorry about the confusion. The BAR is *not* mapped to the kernel yet >> (it is BAR1, there is no BAR3 on GK20A) and an ioremap_*() is >> performed in ttm_bo_ioremap() to make the part of the BAR where the >> buffer is mapped visible. It seems that doing an ioremap_wc() on the >> BAR area on Tegra is what leads to these errors. ioremap() or >> ioremap_nocache() (which are in effect the same on ARM) do not cause >> this issue. >> > It would be cool if you could ask HW, or the blob developers, if this is > a general issue. The external abort is clearly the GPUs AXI client > responding with an error to the read request, though I'm not clear where > a WC read differs from an uncached one. Will check that. > >> The best way to solve this issue would be to not use the BAR at all >> since the memory behind these objects can be directly accessed by the >> CPU. As such it would better be mapped using ttm_bo_kmap_ttm() >> instead. But right now this is clearly not how nouveau_bo.c is written >> and it does not look like this can easily be done. :/ > > Yeah, it sounds like we want this shortcut for stolen VRAM > implementations. Actually, isn't it the case that we do not want to use TTM at all for stolen VRAM (UMA) devices? I am trying to wrap my head around this since a while already, and could not think of a way to use the current TTM-based nouveau_bo optimally for GK20A. Because we cannot do without the idea of VRAM and GART, we will always have to "move" objects from one location to another, or deal with constraints that do not make sense for UMA devices (like in the current case, accessing VRAM objects through the BAR). I am currently contemplating the idea of writing an alternative non-TTM implementation of nouveau_bo for UMA devices, that would (hopefully) be much simpler and would spare us a lot of stunts. On the other hand, this sounds like a considerable work and I would like to make sure that my lack of understanding of TTM is not driving me to the wrong solution. Thoughts? Thanks, Alex.
Am Freitag, den 23.05.2014, 18:43 +0900 schrieb Alexandre Courbot: > On 05/23/2014 06:24 PM, Lucas Stach wrote: > > Am Freitag, den 23.05.2014, 16:10 +0900 schrieb Alexandre Courbot: > >> On Mon, May 19, 2014 at 7:16 PM, Lucas Stach <l.stach@pengutronix.de> wrote: > >>> Am Montag, den 19.05.2014, 19:06 +0900 schrieb Alexandre Courbot: > >>>> On 05/19/2014 06:57 PM, Lucas Stach wrote: > >>>>> Am Montag, den 19.05.2014, 18:46 +0900 schrieb Alexandre Courbot: > >>>>>> This patch is not meant to be merged, but rather to try and understand > >>>>>> why this is needed and what a more suitable solution could be. > >>>>>> > >>>>>> Allowing BOs to be write-cached results in the following happening when > >>>>>> trying to run any program on Tegra/GK20A: > >>>>>> > >>>>>> Unhandled fault: external abort on non-linefetch (0x1008) at 0xf0036010 > >>>>>> ... > >>>>>> (nouveau_bo_rd32) from [<c0357d00>] (nouveau_fence_update+0x5c/0x80) > >>>>>> (nouveau_fence_update) from [<c0357d40>] (nouveau_fence_done+0x1c/0x38) > >>>>>> (nouveau_fence_done) from [<c02c3d00>] (ttm_bo_wait+0xec/0x168) > >>>>>> (ttm_bo_wait) from [<c035e334>] (nouveau_gem_ioctl_cpu_prep+0x44/0x100) > >>>>>> (nouveau_gem_ioctl_cpu_prep) from [<c02aaa84>] (drm_ioctl+0x1d8/0x4f4) > >>>>>> (drm_ioctl) from [<c0355394>] (nouveau_drm_ioctl+0x54/0x80) > >>>>>> (nouveau_drm_ioctl) from [<c00ee7b0>] (do_vfs_ioctl+0x3dc/0x5a0) > >>>>>> (do_vfs_ioctl) from [<c00ee9a8>] (SyS_ioctl+0x34/0x5c) > >>>>>> (SyS_ioctl) from [<c000e6e0>] (ret_fast_syscall+0x0/0x30 > >>>>>> > >>>>>> The offending nouveau_bo_rd32 is done over an IO-mapped BO, e.g. a BO > >>>>>> mapped through the BAR. > >>>>>> > >>>>> Um wait, this memory is behind an already mapped bar? I think ioremap on > >>>>> ARM defaults to uncached mappings, so if you want to access the memory > >>>>> behind this bar as WC you need to map the BAR as a whole as WC by using > >>>>> ioremap_wc. > >>>> > >>>> Tried mapping the BAR using ioremap_wc(), but to no avail. On the other > >>>> hand, could it be that VRAM BOs end up creating a mapping over an > >>>> already-mapped region? I seem to remember that ARM might not like it... > >>> > >>> Multiple mapping are generally allowed, as long as they have the same > >>> caching state. It's conflicting mappings (uncached vs cached, or cached > >>> vs wc), that are documented to yield undefined results. > >> > >> Sorry about the confusion. The BAR is *not* mapped to the kernel yet > >> (it is BAR1, there is no BAR3 on GK20A) and an ioremap_*() is > >> performed in ttm_bo_ioremap() to make the part of the BAR where the > >> buffer is mapped visible. It seems that doing an ioremap_wc() on the > >> BAR area on Tegra is what leads to these errors. ioremap() or > >> ioremap_nocache() (which are in effect the same on ARM) do not cause > >> this issue. > >> > > It would be cool if you could ask HW, or the blob developers, if this is > > a general issue. The external abort is clearly the GPUs AXI client > > responding with an error to the read request, though I'm not clear where > > a WC read differs from an uncached one. > > Will check that. > > > > >> The best way to solve this issue would be to not use the BAR at all > >> since the memory behind these objects can be directly accessed by the > >> CPU. As such it would better be mapped using ttm_bo_kmap_ttm() > >> instead. But right now this is clearly not how nouveau_bo.c is written > >> and it does not look like this can easily be done. :/ > > > > Yeah, it sounds like we want this shortcut for stolen VRAM > > implementations. > > Actually, isn't it the case that we do not want to use TTM at all for > stolen VRAM (UMA) devices? > > I am trying to wrap my head around this since a while already, and could > not think of a way to use the current TTM-based nouveau_bo optimally for > GK20A. Because we cannot do without the idea of VRAM and GART, we will > always have to "move" objects from one location to another, or deal with > constraints that do not make sense for UMA devices (like in the current > case, accessing VRAM objects through the BAR). > > I am currently contemplating the idea of writing an alternative non-TTM > implementation of nouveau_bo for UMA devices, that would (hopefully) be > much simpler and would spare us a lot of stunts. > > On the other hand, this sounds like a considerable work and I would like > to make sure that my lack of understanding of TTM is not driving me to > the wrong solution. Thoughts? > You may want to make yourself aware of all the quirks required for sharing memory between the GPU and CPU on an ARM host. I think there are far more involved than what you see now and writing an replacement for TTM will not be an easy task. Doing away with the concept of two memory areas will not get you to a single unified address space. You would have to deal with things like not being able to change the caching state of pages in the systems lowmem yourself. You will still have to deal with remapping pages that aren't currently visible to the CPU (ok this is not an issue on Jetson right now as it only has 2GB of RAM), because it's in systems highmem, or even in a different LPAE area. You really want to be sure you are aware of all the consequences of this, before considering this task. Regards, Lucas
On 05/23/2014 06:59 PM, Lucas Stach wrote: > Am Freitag, den 23.05.2014, 18:43 +0900 schrieb Alexandre Courbot: >> On 05/23/2014 06:24 PM, Lucas Stach wrote: >>> Am Freitag, den 23.05.2014, 16:10 +0900 schrieb Alexandre Courbot: >>>> On Mon, May 19, 2014 at 7:16 PM, Lucas Stach <l.stach@pengutronix.de> wrote: >>>>> Am Montag, den 19.05.2014, 19:06 +0900 schrieb Alexandre Courbot: >>>>>> On 05/19/2014 06:57 PM, Lucas Stach wrote: >>>>>>> Am Montag, den 19.05.2014, 18:46 +0900 schrieb Alexandre Courbot: >>>>>>>> This patch is not meant to be merged, but rather to try and understand >>>>>>>> why this is needed and what a more suitable solution could be. >>>>>>>> >>>>>>>> Allowing BOs to be write-cached results in the following happening when >>>>>>>> trying to run any program on Tegra/GK20A: >>>>>>>> >>>>>>>> Unhandled fault: external abort on non-linefetch (0x1008) at 0xf0036010 >>>>>>>> ... >>>>>>>> (nouveau_bo_rd32) from [<c0357d00>] (nouveau_fence_update+0x5c/0x80) >>>>>>>> (nouveau_fence_update) from [<c0357d40>] (nouveau_fence_done+0x1c/0x38) >>>>>>>> (nouveau_fence_done) from [<c02c3d00>] (ttm_bo_wait+0xec/0x168) >>>>>>>> (ttm_bo_wait) from [<c035e334>] (nouveau_gem_ioctl_cpu_prep+0x44/0x100) >>>>>>>> (nouveau_gem_ioctl_cpu_prep) from [<c02aaa84>] (drm_ioctl+0x1d8/0x4f4) >>>>>>>> (drm_ioctl) from [<c0355394>] (nouveau_drm_ioctl+0x54/0x80) >>>>>>>> (nouveau_drm_ioctl) from [<c00ee7b0>] (do_vfs_ioctl+0x3dc/0x5a0) >>>>>>>> (do_vfs_ioctl) from [<c00ee9a8>] (SyS_ioctl+0x34/0x5c) >>>>>>>> (SyS_ioctl) from [<c000e6e0>] (ret_fast_syscall+0x0/0x30 >>>>>>>> >>>>>>>> The offending nouveau_bo_rd32 is done over an IO-mapped BO, e.g. a BO >>>>>>>> mapped through the BAR. >>>>>>>> >>>>>>> Um wait, this memory is behind an already mapped bar? I think ioremap on >>>>>>> ARM defaults to uncached mappings, so if you want to access the memory >>>>>>> behind this bar as WC you need to map the BAR as a whole as WC by using >>>>>>> ioremap_wc. >>>>>> >>>>>> Tried mapping the BAR using ioremap_wc(), but to no avail. On the other >>>>>> hand, could it be that VRAM BOs end up creating a mapping over an >>>>>> already-mapped region? I seem to remember that ARM might not like it... >>>>> >>>>> Multiple mapping are generally allowed, as long as they have the same >>>>> caching state. It's conflicting mappings (uncached vs cached, or cached >>>>> vs wc), that are documented to yield undefined results. >>>> >>>> Sorry about the confusion. The BAR is *not* mapped to the kernel yet >>>> (it is BAR1, there is no BAR3 on GK20A) and an ioremap_*() is >>>> performed in ttm_bo_ioremap() to make the part of the BAR where the >>>> buffer is mapped visible. It seems that doing an ioremap_wc() on the >>>> BAR area on Tegra is what leads to these errors. ioremap() or >>>> ioremap_nocache() (which are in effect the same on ARM) do not cause >>>> this issue. >>>> >>> It would be cool if you could ask HW, or the blob developers, if this is >>> a general issue. The external abort is clearly the GPUs AXI client >>> responding with an error to the read request, though I'm not clear where >>> a WC read differs from an uncached one. >> >> Will check that. So after checking with more knowledgeable people, it turns out this is the expected behavior on ARM and BAR regions should be mapped uncached on GK20A. All the more reasons to avoid using the BAR at all. >> >>> >>>> The best way to solve this issue would be to not use the BAR at all >>>> since the memory behind these objects can be directly accessed by the >>>> CPU. As such it would better be mapped using ttm_bo_kmap_ttm() >>>> instead. But right now this is clearly not how nouveau_bo.c is written >>>> and it does not look like this can easily be done. :/ >>> >>> Yeah, it sounds like we want this shortcut for stolen VRAM >>> implementations. >> >> Actually, isn't it the case that we do not want to use TTM at all for >> stolen VRAM (UMA) devices? >> >> I am trying to wrap my head around this since a while already, and could >> not think of a way to use the current TTM-based nouveau_bo optimally for >> GK20A. Because we cannot do without the idea of VRAM and GART, we will >> always have to "move" objects from one location to another, or deal with >> constraints that do not make sense for UMA devices (like in the current >> case, accessing VRAM objects through the BAR). >> >> I am currently contemplating the idea of writing an alternative non-TTM >> implementation of nouveau_bo for UMA devices, that would (hopefully) be >> much simpler and would spare us a lot of stunts. >> >> On the other hand, this sounds like a considerable work and I would like >> to make sure that my lack of understanding of TTM is not driving me to >> the wrong solution. Thoughts? >> > You may want to make yourself aware of all the quirks required for > sharing memory between the GPU and CPU on an ARM host. I think there are > far more involved than what you see now and writing an replacement for > TTM will not be an easy task. > > Doing away with the concept of two memory areas will not get you to a > single unified address space. You would have to deal with things like > not being able to change the caching state of pages in the systems > lowmem yourself. You will still have to deal with remapping pages that > aren't currently visible to the CPU (ok this is not an issue on Jetson > right now as it only has 2GB of RAM), because it's in systems highmem, > or even in a different LPAE area. > > You really want to be sure you are aware of all the consequences of > this, before considering this task. Yep, that's why I am seeking advice here. My first hope is that with a few tweaks we will be able to keep using TTM and the current nouveau_bo implementation. But unless I missed something this is not going to be easy. We can also use something like the patch I originally sent to make it work, although not with good performance, on GK20A. Not very graceful, but it will allow applications to run. In the long run though, we will want to achieve better performance, and it seems like a BO implementation targeted at UMA devices would also be beneficial to quite a few desktop GPUs. So as tricky as it may be I'm interested in gathering thoughts and why not giving it a first try with GK20A, even if it imposes some limitations like having buffers in lowmem in a first time (we can probably live with this one for a short while, and 64 bits will also be coming to the rescue :)) Thanks, Alex.
On 23.05.2014 17:40, Alex Courbot wrote: > On 05/23/2014 06:59 PM, Lucas Stach wrote: > So after checking with more knowledgeable people, it turns out this is > the expected behavior on ARM and BAR regions should be mapped uncached > on GK20A. All the more reasons to avoid using the BAR at all. This is actually specific to Tegra. >> You may want to make yourself aware of all the quirks required for >> sharing memory between the GPU and CPU on an ARM host. I think there are >> far more involved than what you see now and writing an replacement for >> TTM will not be an easy task. >> >> Doing away with the concept of two memory areas will not get you to a >> single unified address space. You would have to deal with things like >> not being able to change the caching state of pages in the systems >> lowmem yourself. You will still have to deal with remapping pages that >> aren't currently visible to the CPU (ok this is not an issue on Jetson >> right now as it only has 2GB of RAM), because it's in systems highmem, >> or even in a different LPAE area. >> >> You really want to be sure you are aware of all the consequences of >> this, before considering this task. > > Yep, that's why I am seeking advice here. My first hope is that with a > few tweaks we will be able to keep using TTM and the current nouveau_bo > implementation. But unless I missed something this is not going to be easy. > > We can also use something like the patch I originally sent to make it > work, although not with good performance, on GK20A. Not very graceful, > but it will allow applications to run. > > In the long run though, we will want to achieve better performance, and > it seems like a BO implementation targeted at UMA devices would also be > beneficial to quite a few desktop GPUs. So as tricky as it may be I'm > interested in gathering thoughts and why not giving it a first try with > GK20A, even if it imposes some limitations like having buffers in lowmem > in a first time (we can probably live with this one for a short while, > and 64 bits will also be coming to the rescue :)) I don't think lowmem or LPAE is any problem, if the memory manager is designed with that in mind. Vast majority of the buffers kernel allocates do not need to be touched in kernel space. Actually I can't think of any buffers that we allocate on behalf of user space that would need to be permanently mapped also to kernel. In case or relocs only push buffer needs to be temporarily mapped to kernel. Ultimately even relocs are not necessary if we expose GPU virtual addresses directly to user space. But that's another topic. Terje
Am Montag, den 26.05.2014, 09:45 +0300 schrieb Terje Bergström: > On 23.05.2014 17:40, Alex Courbot wrote: > > On 05/23/2014 06:59 PM, Lucas Stach wrote: > > So after checking with more knowledgeable people, it turns out this is > > the expected behavior on ARM and BAR regions should be mapped uncached > > on GK20A. All the more reasons to avoid using the BAR at all. > > This is actually specific to Tegra. > > >> You may want to make yourself aware of all the quirks required for > >> sharing memory between the GPU and CPU on an ARM host. I think there are > >> far more involved than what you see now and writing an replacement for > >> TTM will not be an easy task. > >> > >> Doing away with the concept of two memory areas will not get you to a > >> single unified address space. You would have to deal with things like > >> not being able to change the caching state of pages in the systems > >> lowmem yourself. You will still have to deal with remapping pages that > >> aren't currently visible to the CPU (ok this is not an issue on Jetson > >> right now as it only has 2GB of RAM), because it's in systems highmem, > >> or even in a different LPAE area. > >> > >> You really want to be sure you are aware of all the consequences of > >> this, before considering this task. > > > > Yep, that's why I am seeking advice here. My first hope is that with a > > few tweaks we will be able to keep using TTM and the current nouveau_bo > > implementation. But unless I missed something this is not going to be easy. > > > > We can also use something like the patch I originally sent to make it > > work, although not with good performance, on GK20A. Not very graceful, > > but it will allow applications to run. > > > > In the long run though, we will want to achieve better performance, and > > it seems like a BO implementation targeted at UMA devices would also be > > beneficial to quite a few desktop GPUs. So as tricky as it may be I'm > > interested in gathering thoughts and why not giving it a first try with > > GK20A, even if it imposes some limitations like having buffers in lowmem > > in a first time (we can probably live with this one for a short while, > > and 64 bits will also be coming to the rescue :)) > > I don't think lowmem or LPAE is any problem, if the memory manager is > designed with that in mind. Vast majority of the buffers kernel > allocates do not need to be touched in kernel space. > > Actually I can't think of any buffers that we allocate on behalf of user > space that would need to be permanently mapped also to kernel. In case > or relocs only push buffer needs to be temporarily mapped to kernel. > > Ultimately even relocs are not necessary if we expose GPU virtual > addresses directly to user space. But that's another topic. > Nouveau already exposes constant virtual addresses to userspace and skips the pushbuf patching when the presumed offset from userspace is the same as what the kernel thinks it should be. The problem with lowmem on ARM is that you can't unmap those pages from the kernel cached mapping. So if you alloc a page, give it to userspace and userspace decides to map the page WC you just produced a conflicting mapping, which may yield undefined results on ARMv7. You may think this is not a problem as you are not touching the kernel cached mapping, but in fact it is. The CPUs prefetcher can still access this mapping. Although it won't wander over a page boundary for automatic prefetching it may still do this when explicitly instructed to do so by code, which may happen for example if your page happens to be near a kernel list, where the list traversal code includes explicit prefetch instructions. Regards, Lucas
On Mon, May 26, 2014 at 6:21 PM, Lucas Stach <l.stach@pengutronix.de> wrote: > Am Montag, den 26.05.2014, 09:45 +0300 schrieb Terje Bergström: >> On 23.05.2014 17:40, Alex Courbot wrote: >> > On 05/23/2014 06:59 PM, Lucas Stach wrote: >> > So after checking with more knowledgeable people, it turns out this is >> > the expected behavior on ARM and BAR regions should be mapped uncached >> > on GK20A. All the more reasons to avoid using the BAR at all. >> >> This is actually specific to Tegra. >> >> >> You may want to make yourself aware of all the quirks required for >> >> sharing memory between the GPU and CPU on an ARM host. I think there are >> >> far more involved than what you see now and writing an replacement for >> >> TTM will not be an easy task. >> >> >> >> Doing away with the concept of two memory areas will not get you to a >> >> single unified address space. You would have to deal with things like >> >> not being able to change the caching state of pages in the systems >> >> lowmem yourself. You will still have to deal with remapping pages that >> >> aren't currently visible to the CPU (ok this is not an issue on Jetson >> >> right now as it only has 2GB of RAM), because it's in systems highmem, >> >> or even in a different LPAE area. >> >> >> >> You really want to be sure you are aware of all the consequences of >> >> this, before considering this task. >> > >> > Yep, that's why I am seeking advice here. My first hope is that with a >> > few tweaks we will be able to keep using TTM and the current nouveau_bo >> > implementation. But unless I missed something this is not going to be easy. >> > >> > We can also use something like the patch I originally sent to make it >> > work, although not with good performance, on GK20A. Not very graceful, >> > but it will allow applications to run. >> > >> > In the long run though, we will want to achieve better performance, and >> > it seems like a BO implementation targeted at UMA devices would also be >> > beneficial to quite a few desktop GPUs. So as tricky as it may be I'm >> > interested in gathering thoughts and why not giving it a first try with >> > GK20A, even if it imposes some limitations like having buffers in lowmem >> > in a first time (we can probably live with this one for a short while, >> > and 64 bits will also be coming to the rescue :)) >> >> I don't think lowmem or LPAE is any problem, if the memory manager is >> designed with that in mind. Vast majority of the buffers kernel >> allocates do not need to be touched in kernel space. >> >> Actually I can't think of any buffers that we allocate on behalf of user >> space that would need to be permanently mapped also to kernel. In case >> or relocs only push buffer needs to be temporarily mapped to kernel. >> >> Ultimately even relocs are not necessary if we expose GPU virtual >> addresses directly to user space. But that's another topic. >> > Nouveau already exposes constant virtual addresses to userspace and > skips the pushbuf patching when the presumed offset from userspace is > the same as what the kernel thinks it should be. > > The problem with lowmem on ARM is that you can't unmap those pages from > the kernel cached mapping. So if you alloc a page, give it to userspace > and userspace decides to map the page WC you just produced a conflicting > mapping, which may yield undefined results on ARMv7. You may think this > is not a problem as you are not touching the kernel cached mapping, but > in fact it is. The CPUs prefetcher can still access this mapping. Why would this memory be mapped into the kernel? AFAICT Nouveau only maps fences and (somehow) PBs into the kernel. Other BOs are not mapped unless I missed something. Or are you talking about VRAM allocated by dma_alloc_*()? We prevent this from happening by using the CMA allocator (which doesn't create a kmap) directly, which has its own problems (cannot compile Nouveau as a module and use these allocators). In the future we plan to use the iommu to present sparse memory pages in a way the GPU likes.
On Mon, May 26, 2014 at 5:02 PM, Alexandre Courbot <gnurou@gmail.com> wrote: > On Mon, May 26, 2014 at 6:21 PM, Lucas Stach <l.stach@pengutronix.de> wrote: >> Am Montag, den 26.05.2014, 09:45 +0300 schrieb Terje Bergström: >>> On 23.05.2014 17:40, Alex Courbot wrote: >>> > On 05/23/2014 06:59 PM, Lucas Stach wrote: >>> > So after checking with more knowledgeable people, it turns out this is >>> > the expected behavior on ARM and BAR regions should be mapped uncached >>> > on GK20A. All the more reasons to avoid using the BAR at all. >>> >>> This is actually specific to Tegra. >>> >>> >> You may want to make yourself aware of all the quirks required for >>> >> sharing memory between the GPU and CPU on an ARM host. I think there are >>> >> far more involved than what you see now and writing an replacement for >>> >> TTM will not be an easy task. >>> >> >>> >> Doing away with the concept of two memory areas will not get you to a >>> >> single unified address space. You would have to deal with things like >>> >> not being able to change the caching state of pages in the systems >>> >> lowmem yourself. You will still have to deal with remapping pages that >>> >> aren't currently visible to the CPU (ok this is not an issue on Jetson >>> >> right now as it only has 2GB of RAM), because it's in systems highmem, >>> >> or even in a different LPAE area. >>> >> >>> >> You really want to be sure you are aware of all the consequences of >>> >> this, before considering this task. >>> > >>> > Yep, that's why I am seeking advice here. My first hope is that with a >>> > few tweaks we will be able to keep using TTM and the current nouveau_bo >>> > implementation. But unless I missed something this is not going to be easy. >>> > >>> > We can also use something like the patch I originally sent to make it >>> > work, although not with good performance, on GK20A. Not very graceful, >>> > but it will allow applications to run. >>> > >>> > In the long run though, we will want to achieve better performance, and >>> > it seems like a BO implementation targeted at UMA devices would also be >>> > beneficial to quite a few desktop GPUs. So as tricky as it may be I'm >>> > interested in gathering thoughts and why not giving it a first try with >>> > GK20A, even if it imposes some limitations like having buffers in lowmem >>> > in a first time (we can probably live with this one for a short while, >>> > and 64 bits will also be coming to the rescue :)) >>> >>> I don't think lowmem or LPAE is any problem, if the memory manager is >>> designed with that in mind. Vast majority of the buffers kernel >>> allocates do not need to be touched in kernel space. >>> >>> Actually I can't think of any buffers that we allocate on behalf of user >>> space that would need to be permanently mapped also to kernel. In case >>> or relocs only push buffer needs to be temporarily mapped to kernel. >>> >>> Ultimately even relocs are not necessary if we expose GPU virtual >>> addresses directly to user space. But that's another topic. >>> >> Nouveau already exposes constant virtual addresses to userspace and >> skips the pushbuf patching when the presumed offset from userspace is >> the same as what the kernel thinks it should be. >> >> The problem with lowmem on ARM is that you can't unmap those pages from >> the kernel cached mapping. So if you alloc a page, give it to userspace >> and userspace decides to map the page WC you just produced a conflicting >> mapping, which may yield undefined results on ARMv7. You may think this >> is not a problem as you are not touching the kernel cached mapping, but >> in fact it is. The CPUs prefetcher can still access this mapping. > > Why would this memory be mapped into the kernel? On ARM the kernel keeps a linear mapping of lowmem using sections (ARM's version of huge pages). This is always cached, and because the sections are not 4k, it's a pain to remove parts of it. See arch/arm/mm/mmu.c That said, I don't think this issue exists on A15 (which is what those GPUs are paired with), so it's a purely theoretical problem. Stéphane
On Tue, May 27, 2014 at 10:07 AM, Stéphane Marchesin <stephane.marchesin@gmail.com> wrote: > On Mon, May 26, 2014 at 5:02 PM, Alexandre Courbot <gnurou@gmail.com> wrote: >> On Mon, May 26, 2014 at 6:21 PM, Lucas Stach <l.stach@pengutronix.de> wrote: >>> Am Montag, den 26.05.2014, 09:45 +0300 schrieb Terje Bergström: >>>> On 23.05.2014 17:40, Alex Courbot wrote: >>>> > On 05/23/2014 06:59 PM, Lucas Stach wrote: >>>> > So after checking with more knowledgeable people, it turns out this is >>>> > the expected behavior on ARM and BAR regions should be mapped uncached >>>> > on GK20A. All the more reasons to avoid using the BAR at all. >>>> >>>> This is actually specific to Tegra. >>>> >>>> >> You may want to make yourself aware of all the quirks required for >>>> >> sharing memory between the GPU and CPU on an ARM host. I think there are >>>> >> far more involved than what you see now and writing an replacement for >>>> >> TTM will not be an easy task. >>>> >> >>>> >> Doing away with the concept of two memory areas will not get you to a >>>> >> single unified address space. You would have to deal with things like >>>> >> not being able to change the caching state of pages in the systems >>>> >> lowmem yourself. You will still have to deal with remapping pages that >>>> >> aren't currently visible to the CPU (ok this is not an issue on Jetson >>>> >> right now as it only has 2GB of RAM), because it's in systems highmem, >>>> >> or even in a different LPAE area. >>>> >> >>>> >> You really want to be sure you are aware of all the consequences of >>>> >> this, before considering this task. >>>> > >>>> > Yep, that's why I am seeking advice here. My first hope is that with a >>>> > few tweaks we will be able to keep using TTM and the current nouveau_bo >>>> > implementation. But unless I missed something this is not going to be easy. >>>> > >>>> > We can also use something like the patch I originally sent to make it >>>> > work, although not with good performance, on GK20A. Not very graceful, >>>> > but it will allow applications to run. >>>> > >>>> > In the long run though, we will want to achieve better performance, and >>>> > it seems like a BO implementation targeted at UMA devices would also be >>>> > beneficial to quite a few desktop GPUs. So as tricky as it may be I'm >>>> > interested in gathering thoughts and why not giving it a first try with >>>> > GK20A, even if it imposes some limitations like having buffers in lowmem >>>> > in a first time (we can probably live with this one for a short while, >>>> > and 64 bits will also be coming to the rescue :)) >>>> >>>> I don't think lowmem or LPAE is any problem, if the memory manager is >>>> designed with that in mind. Vast majority of the buffers kernel >>>> allocates do not need to be touched in kernel space. >>>> >>>> Actually I can't think of any buffers that we allocate on behalf of user >>>> space that would need to be permanently mapped also to kernel. In case >>>> or relocs only push buffer needs to be temporarily mapped to kernel. >>>> >>>> Ultimately even relocs are not necessary if we expose GPU virtual >>>> addresses directly to user space. But that's another topic. >>>> >>> Nouveau already exposes constant virtual addresses to userspace and >>> skips the pushbuf patching when the presumed offset from userspace is >>> the same as what the kernel thinks it should be. >>> >>> The problem with lowmem on ARM is that you can't unmap those pages from >>> the kernel cached mapping. So if you alloc a page, give it to userspace >>> and userspace decides to map the page WC you just produced a conflicting >>> mapping, which may yield undefined results on ARMv7. You may think this >>> is not a problem as you are not touching the kernel cached mapping, but >>> in fact it is. The CPUs prefetcher can still access this mapping. >> >> Why would this memory be mapped into the kernel? > > On ARM the kernel keeps a linear mapping of lowmem using sections > (ARM's version of huge pages). This is always cached, and because the > sections are not 4k, it's a pain to remove parts of it. See > arch/arm/mm/mmu.c Ah, are we talking about the directly-mapped low memory region starting at PAGE_OFFSET? Ok, it makes sense now, thanks. But it seems to me that such different mappings can also happen in many other scenarios as well, don't they? How is the issue handled in these cases?
On Mon, May 26, 2014 at 7:42 PM, Alexandre Courbot <gnurou@gmail.com> wrote: > On Tue, May 27, 2014 at 10:07 AM, Stéphane Marchesin > <stephane.marchesin@gmail.com> wrote: >> On Mon, May 26, 2014 at 5:02 PM, Alexandre Courbot <gnurou@gmail.com> wrote: >>> On Mon, May 26, 2014 at 6:21 PM, Lucas Stach <l.stach@pengutronix.de> wrote: >>>> Am Montag, den 26.05.2014, 09:45 +0300 schrieb Terje Bergström: >>>>> On 23.05.2014 17:40, Alex Courbot wrote: >>>>> > On 05/23/2014 06:59 PM, Lucas Stach wrote: >>>>> > So after checking with more knowledgeable people, it turns out this is >>>>> > the expected behavior on ARM and BAR regions should be mapped uncached >>>>> > on GK20A. All the more reasons to avoid using the BAR at all. >>>>> >>>>> This is actually specific to Tegra. >>>>> >>>>> >> You may want to make yourself aware of all the quirks required for >>>>> >> sharing memory between the GPU and CPU on an ARM host. I think there are >>>>> >> far more involved than what you see now and writing an replacement for >>>>> >> TTM will not be an easy task. >>>>> >> >>>>> >> Doing away with the concept of two memory areas will not get you to a >>>>> >> single unified address space. You would have to deal with things like >>>>> >> not being able to change the caching state of pages in the systems >>>>> >> lowmem yourself. You will still have to deal with remapping pages that >>>>> >> aren't currently visible to the CPU (ok this is not an issue on Jetson >>>>> >> right now as it only has 2GB of RAM), because it's in systems highmem, >>>>> >> or even in a different LPAE area. >>>>> >> >>>>> >> You really want to be sure you are aware of all the consequences of >>>>> >> this, before considering this task. >>>>> > >>>>> > Yep, that's why I am seeking advice here. My first hope is that with a >>>>> > few tweaks we will be able to keep using TTM and the current nouveau_bo >>>>> > implementation. But unless I missed something this is not going to be easy. >>>>> > >>>>> > We can also use something like the patch I originally sent to make it >>>>> > work, although not with good performance, on GK20A. Not very graceful, >>>>> > but it will allow applications to run. >>>>> > >>>>> > In the long run though, we will want to achieve better performance, and >>>>> > it seems like a BO implementation targeted at UMA devices would also be >>>>> > beneficial to quite a few desktop GPUs. So as tricky as it may be I'm >>>>> > interested in gathering thoughts and why not giving it a first try with >>>>> > GK20A, even if it imposes some limitations like having buffers in lowmem >>>>> > in a first time (we can probably live with this one for a short while, >>>>> > and 64 bits will also be coming to the rescue :)) >>>>> >>>>> I don't think lowmem or LPAE is any problem, if the memory manager is >>>>> designed with that in mind. Vast majority of the buffers kernel >>>>> allocates do not need to be touched in kernel space. >>>>> >>>>> Actually I can't think of any buffers that we allocate on behalf of user >>>>> space that would need to be permanently mapped also to kernel. In case >>>>> or relocs only push buffer needs to be temporarily mapped to kernel. >>>>> >>>>> Ultimately even relocs are not necessary if we expose GPU virtual >>>>> addresses directly to user space. But that's another topic. >>>>> >>>> Nouveau already exposes constant virtual addresses to userspace and >>>> skips the pushbuf patching when the presumed offset from userspace is >>>> the same as what the kernel thinks it should be. >>>> >>>> The problem with lowmem on ARM is that you can't unmap those pages from >>>> the kernel cached mapping. So if you alloc a page, give it to userspace >>>> and userspace decides to map the page WC you just produced a conflicting >>>> mapping, which may yield undefined results on ARMv7. You may think this >>>> is not a problem as you are not touching the kernel cached mapping, but >>>> in fact it is. The CPUs prefetcher can still access this mapping. >>> >>> Why would this memory be mapped into the kernel? >> >> On ARM the kernel keeps a linear mapping of lowmem using sections >> (ARM's version of huge pages). This is always cached, and because the >> sections are not 4k, it's a pain to remove parts of it. See >> arch/arm/mm/mmu.c > > Ah, are we talking about the directly-mapped low memory region > starting at PAGE_OFFSET? Ok, it makes sense now, thanks. > > But it seems to me that such different mappings can also happen in > many other scenarios as well, don't they? How is the issue handled in > these cases? It depends. A lot of cache controllers actually implement a solution for that in hardware, in the cache controller. For example I think Tegra2 is one of those platforms. And then a lot of platforms just ignore the issue completely because it has very low probability. Stéphane
diff --git a/drivers/gpu/drm/nouveau/nouveau_bo.c b/drivers/gpu/drm/nouveau/nouveau_bo.c index 8db54a217232..9cfb8e61f5c4 100644 --- a/drivers/gpu/drm/nouveau/nouveau_bo.c +++ b/drivers/gpu/drm/nouveau/nouveau_bo.c @@ -552,7 +552,11 @@ nouveau_bo_init_mem_type(struct ttm_bo_device *bdev, uint32_t type, TTM_MEMTYPE_FLAG_MAPPABLE; man->available_caching = TTM_PL_FLAG_UNCACHED | TTM_PL_FLAG_WC; +#if defined(__arm__) + man->default_caching = TTM_PL_FLAG_UNCACHED; +#else man->default_caching = TTM_PL_FLAG_WC; +#endif break; case TTM_PL_TT: if (nv_device(drm->device)->card_type >= NV_50)
This patch is not meant to be merged, but rather to try and understand why this is needed and what a more suitable solution could be. Allowing BOs to be write-cached results in the following happening when trying to run any program on Tegra/GK20A: Unhandled fault: external abort on non-linefetch (0x1008) at 0xf0036010 ... (nouveau_bo_rd32) from [<c0357d00>] (nouveau_fence_update+0x5c/0x80) (nouveau_fence_update) from [<c0357d40>] (nouveau_fence_done+0x1c/0x38) (nouveau_fence_done) from [<c02c3d00>] (ttm_bo_wait+0xec/0x168) (ttm_bo_wait) from [<c035e334>] (nouveau_gem_ioctl_cpu_prep+0x44/0x100) (nouveau_gem_ioctl_cpu_prep) from [<c02aaa84>] (drm_ioctl+0x1d8/0x4f4) (drm_ioctl) from [<c0355394>] (nouveau_drm_ioctl+0x54/0x80) (nouveau_drm_ioctl) from [<c00ee7b0>] (do_vfs_ioctl+0x3dc/0x5a0) (do_vfs_ioctl) from [<c00ee9a8>] (SyS_ioctl+0x34/0x5c) (SyS_ioctl) from [<c000e6e0>] (ret_fast_syscall+0x0/0x30 The offending nouveau_bo_rd32 is done over an IO-mapped BO, e.g. a BO mapped through the BAR. Any idea about the origin of this behavior? Does ARM forbid cached mappings over IO regions? Signed-off-by: Alexandre Courbot <acourbot@nvidia.com> --- drivers/gpu/drm/nouveau/nouveau_bo.c | 4 ++++ 1 file changed, 4 insertions(+)